Automated material handling systems are used in a variety of industries to move various materials from one location to another location. Semiconductor fabrication facilities, in particular, commonly utilize automated material handling systems for fabricating integrated circuits on semiconductor wafers.
A conventional semiconductor fabrication plant typically includes multiple fabrication areas or bays interconnected by a path, such as a conveyor belt. Each bay generally includes the requisite fabrication tools (interconnected by a subpath) to process semiconductor wafers for a particular purpose, such as photolithography, chemical-mechanical polishing, or chemical vapor deposition, for example. Material stockers or stocking tools generally lie about the plant and store semiconductor wafers waiting to be processed. The wafers are usually stored in cassettes each of which typically hold up to 25 wafers. A lot is a logical grouping of wafers in a cassette. The lot can migrate from cassette to cassette as it progresses through the chain of production processes Each material stocker typically services two or more bays and can hold hundreds of cassettes.
Once a lot has been retrieved, and the equipment has been set up, the operation on the wafers by a particular piece of equipment, or "tool," can begin. At this point, the lot is moved-in to the operation. This state is indicated to the host application by the operator for the lot. The lot remains in this state until the operation is completed. Once the operation is completed, the operator must perform tests and verifications on the wafers. When all tests and verifications have been performed, the host computer application program must be notified. Wafers may have moved from one cassette to another as a result of the operation. The host application has to be notified of this. The operator then places the cassette of "moved-out" wafers in the material stocker, to await orders as to the location of the next piece of equipment that will perform operations on the wafers.
Some present material handling systems exhibit excess movement of materials through the system that negatively impacts manufacturing throughput. In an example system, there are multiple locations that are available for a particular manufacturing process step. However, the system only provides mappings for one location per manufacturing process step. Therefore, the system does not know that there are alternate locations available for performing the processing step. When the AMHS dispatches material to a particular location, and the stocker associated with that location is unavailable, the material is redirected to an adjacent stocker or temporary storage. If other locations are available to perform the processing step, a move request is required to transfer the material from a stocker, such as the adjacent stocker, to the desired location. The AMHS then moves the material to the stocker at the specified location, and the material is then removed and processed.
The semiconductor fabrication plant, including the bays, material stockers and the interconnecting path, typically operates under control of a distributed computer system running a factory management program, such as WorkStream Open sold by Consilium, Inc. In this environment, the automated material handling system (AMHS) may conceptually include the cassettes, the transportation system (e.g., paths) and control system (e.g., the distributed computer system). You may also have an empty carriers management system as well as a separate test wafer management system that form part of the AMHS. Currently these are handled as subsystems and must be manually coordinated by the operators on the line, which has led to delays in the system in processing the wafers. It would be desirable, therefore, to reduce processing delays and increase throughput efficiency by pre-positioning material at the different stocker locations through the use of current automated material handling systems.